GCA is a rare autoimmune disorder and is the most common type of inflammation of medium- to large-sized blood vessels in the body. It usually only occurs in older adults. The most common symptoms of GCA include headache, pain in the shoulders and hips (polymyalgia rheumatica), pain in the jaw (jaw claudication), fever, and blurred vision. Organ-specific markers of injury or damage as well as direct markers of vascular damage and inflammation are currently used by clinicians to assess GCA disease progression; however, these markers are not very useful in guiding treatment. There are also blood tests that clinicians use to monitor GCA activity, such as erythrocyte sedimentation rate (ESR) and C-reactive protein (CRP), but these tests lack specificity and sensitivity. Most treatments available now for GCA are toxic, therefore if other markers indicating disease activity can be found, it may lead to the development of less toxic treatments. This study will use new scientific methods to identify new biomarkers that can be used to monitor disease activity in GCA patients. These biomarkers may be used to help direct clinical care for GCA patients and assist in future drug development.